Problems with nf_nat_ftp.ko and nf_conntrack_ftp.ko in 2.6.22.6

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Hi all,
My 2.6.22.6 Linux box is acting as a NAT device. I found that a NATted FTP client is having problem using active mode to connect to a outside FTP server. (Passive mode works fine.)

From the trace I could see that the PORT command from the FTP client is
correctly modified by the Linux box to use the converted NAT address. However, the confirmation from the server never makes it to the client and the client just keeps retransmitting the PORT command packet.

The interesting part is that active mode can work if the length of the actual IP address of the client is the same as the length of the converted NAT address. It looks like if there is no TCP sequence number modification by the Linux box, the FTP connection can work properly in active mode. I am suspecting that there may a problem in the TCP sequence number tracking in the kernel modules.

The same settings work fine when I try with Linux 2.6.15 loading ip_nat_ftp.ko and ip_conntrack_ftp.ko. Did I miss anything in configuring the Linux 2.6.22.6 box?

Regards,
Ron
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